The present invention relates to the field of hydraulics. More particularly, this invention relates to a xe2x80x9cservolessxe2x80x9d assist mechanism for altering the forces required to position the swashplate of a hydrostatic unit such as a pump or motor. In general, the mechanism can be used to reduce the force and energy levels required to position the swashplate in axial piston pumps and motors. The mechanism is particularly useful in applications where operator xe2x80x9cfeelxe2x80x9d is important, allowing the operator to feel feedback from the vehicle but at reduced force levels. The mechanism provides a dynamic or variable method of affecting or tuning net swashplate moments.
Hydrostatic transmissions have been used in skid steer loaders for a number of years now. In the early days of hydrostatically propelled skid steer loaders, the machines were relatively small and therefore the operator could manually directly control the position of the swashplate and the resulting displacement of the hydraulic unit through mechanical linkage with minimal force and fatigue. As the machines have become larger in recent years, the power and force levels have become too large for the operator to handle without tiring when operating the machine for an extended period of time. Servo-controlled transmissions were developed to overcome the operator fatigue problem, but the operators then felt xe2x80x9cdisconnectedxe2x80x9d from the machine when attempting to control its displacement or swashplate position. The servo control devices require additional power and suffer reduced response capability, especially when response is needed most such as when the machine is near neutral, has low displacement, or is inching.
Various tiltable swashplate arrangements are known for varying displacement in axial piston pumps and motors. In one arrangement, the swashplate has opposite cylindrical trunnions that pivotally mount or journal it in the pump or motor housing. A plurality of pistons slidably mount in corresponding piston bores or chambers arranged in a circular pattern in a rotatable cylinder block that is urged by a block spring toward the tiltable swashplate. A valve plate engages the end of the cylinder block that is remote from the swashplate. Slippers swivelingly attached to the pistons engage a running surface on the swashplate as the cylinder block rotates. If the running surface of the swashplate is perpendicular to the longitudinal axes of the pistons, the pistons do not reciprocate in the cylinder block and no fluid is displaced or consumed by the hydraulic unit. A lubrication hole typically extends longitudinally through the piston and slipper so that oil from the piston bore or chamber can reach the slipper running surface of the swashplate.
When the swashplate is forcibly tilted away from perpendicular, the pistons reciprocate in the piston bores as the pistons are driven in a circle against the inclined plane. This reciprocating action means that the chambers of the pistons on one region of the swashplate are under high pressure, while the piston chambers on the opposite region of the swashplate are under low pressure. Each piston bore or chamber in the cylinder block has a xe2x80x9cpressure profilexe2x80x9d associated with it as the block rotates. The pressure times the area translates into a force, which yields a moment on the swashplate. To move or maintain the swashplate tilted to given degree, a moment of equal and opposite magnitude must be maintained on the swashplate. The operator does this manually by applying a force on a lever or torque on a handle attached to the swashplate or through a conventional servo mechanism. If a servo mechanism is used, operator xe2x80x9cfeelxe2x80x9d is usually lost.
One common method of fine tuning or affecting swashplate moments in a hydrostatic unit is a static method involving designing a specific valve plate with a specific fixed porting configuration to achieve the desired swashplate moments. A valve plate is a substantially flat disc-shaped annular ring of material that is fixed against rotation on the end cap of the hydraulic unit adjacent the rear surface of the rotating cylinder block (which is opposite of the swashplate). The conventional valve plate typically has an arcuate inlet port and an arcuate outlet port formed therethrough on opposing sides of a median axis. These ports reside along arcs that generally align with the pitch circle of the piston bores in the cylinder block. Thus, the inlet and outlet ports generally register with the circular path of the reciprocating pistons as the pistons rotate with the cylinder block against the valve plate. The inlet and outlet ports are angularly spaced apart in the areas or zones where the reciprocating pistons change their direction of reciprocal movement or transition from high pressure to low pressure and vice versa. The top dead center (TDC) and bottom dead center (BDC) positions of the reciprocating pistons generally correspond to these transition zones. The spacing of the inlet and outlet ports of the valve plate depends to some extent on the number of pistons in the rotating cylinder block assembly. Some existing valve plates utilize specially shaped notches, such as xe2x80x9crat tailsxe2x80x9d or xe2x80x9cfish tails,xe2x80x9d at the entrance and/or exit of the ports (i.e.xe2x80x94in the transition zones) to affect the swashplate moments. Moon et al. U.S. Pat. No. 3,585,900 teaches the basics of utilizing valve plate fish tails to affect swashplate moments in axial piston hydraulic units. U.S. Pat. No. 4,550,645 teaches some additional geometric configurations for fish tails and valve plates. Unfortunately, many different valve plates are required to satisfy the swashplate moment demands of the various users. Thus, the number of valve plate designs tends to proliferate and it can be costly to produce and warehouse an adequate selection of valve plates. Furthermore, if a change in swashplate moments is desired, the user must physically disassemble the unit and change the valve plate. Finally, the valve plate configuration is essentially constant or static once a particular valve plate is selected and installed. A valve plate configuration may have beneficial effects on the swashplate moments, performance and controllability of the unit at under certain operating conditions (including but not limited to speed, pressure and displacement), but the same valve plate configuration may have undesirable effects under other conditions within the normal operating range of the unit. Since the valve plate geometry is fixed based upon the valve plate chosen, the user must accept the tradeoffs involved. Careful and elaborate optimization analysis is often required to determine the best valve plate design for the task.
Thus, there is a need for dynamic rather than static means and methods for affecting swashplate moments. There is also a need for a means and method for affecting swashplate moments that does not necessarily involve valve plate design changes or valve plate proliferation.
Therefore, a primary objective of the present invention is the provision of a dynamic means and method for affecting swashplate moments in a hydraulic unit.
Another objective of this invention is the provision of a variable means of affecting swashplate moments throughout the normal operating range of operating conditions of the hydraulic unit.
Another objective of this invention is the provision of a means for reducing net swashplate moments in a manually controlled hydraulic unit to reduce operator fatigue without sacrificing the feel of operator feedback.
Another objective of this invention is the provision of a means for generating a control error signal to a variable orifice valve for bleeding fluid between adjacent pistons to affect bore pressure and subsequently swashplate moments.
Another objective of this invention is the provision of means for varying swashplate moments without the need for changing valve plates in a hydraulic unit.
These and other objectives will be apparent from the drawings, as well as from the description and claims that follow.
The present invention relates to a swashplate assist mechanism for dynamically varying swashplate moments in a multiple piston hydraulic unit. The mechanism includes a valve means disposed in the swashplate and defining an adjustable variable orifice for metering fluid from at least one of the pistons; and means for generating a control error signal to the valve means so as to adjust the size of the variable orifice based upon the control error signal.
This invention provides a plurality of holes in the swashplate running surface and fluidly connects them to a spool valve to meter high-pressure fluid from a leading piston to a trailing piston near one or more of the pressure transition zones so as to reduce swashplate moments.
In one embodiment, two pairs of angularly spaced holes are provided at or near the transition areas at top dead center (TDC) and/or bottom dead center (BDC) of the piston""s reciprocation. A canned spring arrangement connects the control handle to the swashplate so as to yield a control error that is a function of the torque on the swashplate once the preload on the canned spring is exceeded. The control error signal is then transmitted to variable orifice valves that meter oil between a leading piston and a trailing piston to affect the swashplate moments of the unit. The valve can take many forms, including the three-position, three-way spool valve disclosed herein. The invention is adaptable to either manually controlled or servo-assisted units.